1. Field of the Invention
The present invention relates to the art of taking an image of a glossy convex or part-spherical projection, such as a solder ball of a flip chip or a reference mark of a printed circuit board, which projects from a plane, such as a surface of the flip chip or the circuit board.
2. Related Art Statement
Generally, solder balls of a flip chip or reference marks of a printed circuit board ("PCB") are made of solder in the form of a projection having a part-spherical shape smaller than a hemi-spherical shape, or a shape approximate to the part-spherical shape (hereinafter, generally referred to as the "convex" projection). A flip chip has a number of solder balls located at a number of grid points on its back surface, that is, "a grid array of solder balls". A PCB has reference marks or "solder bulged spots" located in its corners.
When a flip chip is mounted on a circuit substrate, first, the flip chip is held by a component holder such as a suction nozzle, and is conveyed by the same to above a flip-flop-mounting place on the circuit substrate. Subsequently, the component holder and/or the circuit substrate are moved toward each other, for mounting the flip chip on the circuit substrate. It is desirable that the flip chip be mounted at an accurate position on the circuit substrate. To this end, it has been practiced that the image of the grid array of solder balls of the flip chip held by the component holder is taken by an image taking device, position errors of the flip chip held by the component holder are calculated based on the taken image, and the component holder and/or the circuit substrate are positioned relative to each other for eliminating the calculated errors.
In order that a circuit component is mounted at an accurate position on a PCB, it is desirable not only that position errors of the circuit component held by a component holder be measured and corrected but also that position errors of the PCB positioned by a positioning device be measured and corrected. To this end, it has been practiced that reference marks are provided on the PCB, the images of the reference marks are taken, and the position errors of the PCB are measured based on the taken images.
However, it is not easy to take the image of a solder ball or a reference mark, because the solder ball or the reference mark is formed of solder and accordingly the outer surface thereof has a gloss. When the solder ball or the reference mark is exposed to light, it is likely that some part of the solder ball or the reference mark intensely reflects the light. Thus, it is difficult to take a clear image of the contour or profile of the solder ball or the reference mark. To solve this problem, it has been a demand for the art of irradiating, as uniformly as possible, the whole surface of a solder ball or a reference mark. However, no satisfactory solution has been obtained.
The above discussion has been made for the art of taking the image of a solder ball or a reference mark by way of example. This is also the case with the art of taking the images of other glossy convex projections.